The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Emissions control is one factor in engine design and engine control. One particular emission, NOx, is a known by-product of combustion. NOx is created by nitrogen and oxygen molecules present in engine intake air disassociating in the high temperatures of combustion, and rates of NOx creation include known relationships to the combustion process, for example, with higher rates of NOx creation being associated with higher combustion temperatures and longer exposure of air molecules to the higher temperatures.
NOx molecules, once created in the combustion chamber, can be converted back into nitrogen and H2O molecules in exemplary devices known in the art within the broader category of aftertreatment devices. Aftertreatment devices are known, for instance, utilizing chemical reactions to treat an exhaust gas flow. One exemplary device includes a selective catalytic reduction device (SCR). An SCR utilizes a reductant capable of reacting with NOx to treat the NOx. One exemplary reductant is ammonia derived from urea injection. A number of alternative reductants are known in the art. Ammonia stored on a catalyst bed within the SCR reacts with and treats NOx.
According to one embodiment of aftertreatment system operation, an engine can be operated in a stoichiometry operating mode or lean combustion mode. Such operation produces NOx and creates conditions in the exhaust gas flow that are known in the art to be unfavorable to treatment of the NOx while the operation remains in the lean combustion mode. A lean NOx trap (LNT) can be used for during such operation to store NOx within the LNT until a purge cycle under stoichiometric or rich combustion can occur to create conditions favorable to treatment of the stored NOx.